Clinical and experimental evidence supports a role for cardiac glycosides (CGs) as potential novel anticancer drugs. However, there are no studies reporting the effect of CGs on the inflammatory tumor microenvironment (TME), which plays a central role in tumor progression and invasiveness. We investigated whether digitoxin affects a) specific pathways involved in motility and/or activation of different cell types shaping TME, and b) cancer cell growth and invasiveness in response to TME-associated factors. To test our hypothesis, conditioned media (CM) from polarized macrophages, and apoptotic or non-apoptotic ovarian cancer cells (SKOV3) were tested as chemoattractants for endothelial cells, monocytes and cancer cells.
We demonstrated that CM from M1 (LPS/IFNγ) and M2 (IL-4/IL-13) polarized macrophages, which mimic inflammatory TME, increased both HUVEC migration and tubularization. Treatment of HUVECs with digitoxin at concentrations within its plasma therapeutic range counteracted these effects. Digitoxin affected the expression of neither M1 (CD80/CD68) nor M2 (CD206/CD163) activation markers, nor the amount of cell-bound IL-1β and CCL22. Accordingly, HUVEC migration in response to CM from digitoxin-treated activated macrophages was unchanged. These data point to a direct effect of digitoxin on HUVEC signaling rather than on the modulation of the cytokine profile released from activated macrophages. At variance with what observed for HUVECs, digitoxin did not prevent monocyte migration induced by SKOV3 CM. In addition, digitoxin significantly impaired SKOV3 growth and migration in response to M1 or M2 macrophage CM. Finally, we showed that digitoxin inhibited FAK phosphorylation in SKOV3 but not PYK2 phosphorylation in monocytes, thus providing a molecular mechanism accounting for the observed differential anti-migratory effect. Overall, digitoxin counteracted salient features of the inflammatory ovarian cancer microenvironment, laying the ground for potential digitoxin repositioning as an anticancer drug.